Technical Field
The present invention relates to an organic electroluminescent material and an organic electroluminescent device and, in particular, to an organic electroluminescent material containing benzimidazole and an organic electroluminescent device.
Related Art
With the advances in electronic technology, a light weight and high efficiency flat display device has been developed. An organic electroluminescent display possibly becomes the mainstream of the next generation flat panel display device due to its advantages of self-luminosity, no restriction on viewing angle, power conservation, simple manufacturing process, low cost, high response speed, full color and so on.
In general, an organic electroluminescent device includes an anode, an organic luminescent layer and a cathode. When a direct current is provided for the organic electroluminescent device, electron holes and electrons flow into the organic luminescent layer respectively through the anode and the cathode. Charge carriers move and then recombine in the organic luminescent layer because of the potential difference caused by an applied electric field. The excitons generated by the recombination of the electrons and the electron holes may excite the luminescent molecules in the organic luminescent layer. The excited luminescent molecules then release the energy in the form of light.
Nowadays, organic electroluminescent displays usually adopt host-guest emitter systems. The organic luminescent layer disposed therein includes a host material and a guest material. Electron holes and electrons transmit to the host material to perform recombination and then generate energy. The guest material can be categorized into fluorescent material and phosphorescent material. Theoretically, the internal quantum efficiency can approach 100% by using appropriate phosphorescent materials. Therefore, the phosphorescent materials recently have become one of the most important developments in the field of organic electroluminescent materials.
In the development of blue host materials, the triplet energy gap of the host materials must be higher than or equal to that of the guest materials to avoid the energy lost caused by reverse energy transfer. The energy lost can result in low luminous efficiency (i.e., low current efficiency) and short emission lifetime. Therefore, it is necessary for the host materials to have greater triplet energy gap. In order to increase the triple energy gap of the blue host materials, much research has been focused on the single benzene ring with various ortho-substituted groups. In ortho-substitution with electron-transporting group (e.g., OXD or TAZ) and hole-transporting group (e.g., Cbz), a bipolar molecule is created by interrupted π-conjugated molecules due to steric hindrance.
Besides, the selection of organic electroluminescent material is not only based on the matching energy gap but also the high temperature of decomposition to avoid pyrolysis caused by high temperature and also avoid the resulted decreasing of stability.
Accordingly, the present invention is provided an organic electroluminescent material containing benzimidazole and an organic electroluminescent device which has high triplet energy gap and fine thermal stability.